A railway track has several properties that need to be monitored to ensure safe travel, some of them geometrical and some of them structural. Certainly there are links between structural and geometrical parameters. Track geometry quality is a set of parameters that describe current geometry of the track such as vertical and lateral irregularities/alignment (vertical alignment is often referred to as “surface” in the USA and “longitudinal level” in Europe), track gauge, cross level and curvature. In the remaining part of the text the term “geometrical parameter(s)” is used for vertical and lateral irregularities/alignment.
Track geometry quality is measured with track recording cars, or by unattended systems fitted on ordinary cars. Measurement frequency can range from e.g. 1-20 times per year depending on safety regulations and maintenance management strategy.
Examples of structural parameters are track stiffness/modulus (both vertical and lateral); clamping force of the fastener between sleeper and rail; stress free temperature of the rails and shear wave velocity of the soil. All these parameters influence the deflection shape of the rail under a given load.
Industrial Relevance and Prior Art of Stress Free Temperature (SFT) and Lateral Stiffness/Resistance
Track buckling is formation of large lateral misalignments in railway track, sometimes resulting in train derailments. Buckles are typically caused by a combination of three major factors: high compressive forces, weakened track conditions, and vehicle loads (train dynamics).
Compressive forces result from stresses induced in a constrained rail by temperatures above its “stress free” state, and from mechanical sources such as train braking and acceleration.
The temperature of the rail at the stress-free state is known as the stress free temperature (SFT) (i.e. the temperature at which the rail experiences zero longitudinal force). Initially, the rail's installation temperature or anchoring temperature is the rail's SFT. Hence, at rail temperatures above the neutral, compressive forces are generated, and at temperatures below the neutral, tensile forces are developed. Track maintenance practices address the high thermal load problem by anchoring the rail at a (neutral) temperature of 10-40° C. depending on yearly average temperature. SFT may change over time due to for example track maintenance, geometrical track degradation and lateral track shift in curves.
Weakened track conditions impacting the tracks buckling potential include: reduced track resistance, lateral alignment defects, and lowered rail SFT. Track resistance is the ability of the ballast, sleepers and fasteners to provide lateral and longitudinal strength to maintain track stability. Resistance is lowered if ballast is missing from under or between the sleepers, or from the ballast shoulder. A full ballast section is important, especially in curves. Track resistance is lowered when ballast is disturbed. Tamping (surfacing), sleeper renewal and undercutting operations will weaken ballast resistance to a great extent. Providing longitudinal resistance to the rail/sleeper structure by adequate rail anchoring is important to prevent rail running and hence the decrease of rail neutral temperature.
To prevent track buckling, SFT and track resistance have to be monitored. Currently there exist a couple of methods to monitor SFT e. g.                Cut-method (The rail is cut and the gap is an estimate of SFT). This is a destructive method, a new weld is needed.        A method wherein fasteners are released and rail lifted. Lifting force is proportional to SFT        
Common to most of the prior art methods is that measurements are taken in one position at a time. This makes the methods time consuming and hence interval between measurements may be stretched (both in time and position along the track).
SE534724C2 describes a continuous method to estimate SFT and track resistance from measurement of track geometry and rail temperature. Two sets of measurements are used from different occasions in order to have a temperature difference.
The present invention is different in that only one measurement at one rail temperature is needed.
U.S. Pat. No. 5,386,727 describes an ultrasonic based method for determining the longitudinal stress in a rail section based on the alteration of an ultrasonic signal transmitted through said rail.
Industrial Relevance and Prior Art Relating to Fastener Clamping Force
In ordered to keep a continuously welded rail in place at correct track gauge, the rails are clamped to the sleepers with a fastener system. Many fastener systems use an elastic clip which holds the rail with a certain force. Sometimes the clamping force may be reduced and the clip may even break. If consecutive clips are missing there may be a safety issue with train derailment as the worst scenario.
One important property of the fastener is that it increases the rail bending stiffness.
Missing clips are traditionally monitored by manual visual inspection. To date there exist a couple of automated systems based on cameras and image processing to find missing clips.
Industrial Relevance and Prior Art of Wheel-Rail Contact Force Measurement
Wheel-rail contact force measurements are used in various applications. Such measurements can be used to find discontinuities in the rail such as a sharp edge at a weld or at the crossing nose of a turnout. It is also often used in the homologation process of new railway vehicles in order to prove safe and comfortable ride and to restrict train-track interaction forces within certain limits.
Wheel-rail contact forces can be measured with strain gauges mounted on the wheels. Also load cells and/or accelerometers mounted in the wheel-set or bogie can be used in different configurations.
Industrial Relevance and Prior Art of Track Stiffness and Track Bed Modulus Measurement
Track stiffness and track bed modulus describe how much the track deflects at a given load. Track deflection needs to be within certain limits. Swift changes of track stiffness along the track can often explain maintenance problems.
SE535848C2 describes a continuous method to determine track stiffness/deflection using track geometry quality parameters measured from a track recording car. Two different measurement systems for track geometry quality are used and by comparing them deflection can be found.
U.S. Pat. No. 6,119,353 describes a continuous method to determine track deflection using laser Doppler technique.
US2006144129 discloses a noncontact measurement system for measuring the vertical stiffness of a railway track. The system comprises first and second optical emitters which are mounted to a measuring vehicle and configured to emit beams of light that are detectable on the underlying surface. A camera is mounted to the vehicle for recording the distance between the beams of light as the vehicle travels along the surface. The distance between the beams of light, which is a function of the surface stiffness, is then measured using image recognition techniques.
Industrial Relevance and Prior Art of Critical Speed Determination
Under certain condition of soft soil and high travelling speed of trains (or airplanes take-off or landing on runways) a high-speed phenomenon can take place. When the speed approaches or exceeds the critical wave velocity for the compound track-ground structure, the track response dramatically changes characteristics. Propagating chock waves are generated by the moving load. This causes extensive vibration and large deflection of the ground. The short term solution is to restrict higher speed through affected areas. In order to resolve the problem, different methods to strengthen the soil can be used.
Current methods to detect and quantify high-speed vibration phenomenon include e.g. geo-dynamic testing to determine the soil shear-wave velocity and stiffness/modulus as well as measurement of vibration when a high-speed train is passing. All current methods though, instrument the track and/or soil at a specific location and cannot be used on a running train to monitor larger distances.